For the use of a coupling of this kind in the drive train of a motor vehicle the current consumption and the constructional size are critical and must thus to be minimized. In addition to this further demands arise: A range of regulation of the torque to be transmitted that is so wide that, on the one hand, slip-free starting from stop and, on the other hand, full separation (also) for noise reasons is possible and finally rapid response in order to be compatible with electronic dynamic drive regulating systems (ESB, ABS, etc.).
Thus a clutch of the initially named kind is known from U.S. Pat. No. 5,845,753 in which the yoke extends from an end face on one side of the clutch externally surrounding latter up to a second end face at the other side of the clutch. This not only increases the diameter and weight but also signifies a large mass to be magnetized which consumes much current and leads to the breaking down of the magnetic field prior to release of the clutch taking too long for a useful control in the drive train of a motor vehicle. Moreover, the magnetic field lines which pass through the space filled with the magnetorheological fluid are of low density and very irregularly distributed.
Furthermore, a clutch of the said kind is known from EP 940 286 A2 (FIGS. 5 and 6) in which two yokes, of which one is provided with a magnet coil form an end face parallel to the discs on both sides of the clutch. The magnetic field passes through the discs and indeed in the external region in a magnetic flux direction and in the interior region in the opposite direction. Correspondingly the discs are separated from one another by zones of small magnetic permeability extending in the peripheral direction in order to prevent a magnetic short circuit. These zones could be provided by complicated and thus costly metallurgical measures. In practice there are however slits which weaken the discs in the peripheral direction and in the centrifugal direction. That is undesirable for a clutch in the drive train of a motor vehicle.